Field of the Invention
The present invention relates to a wavelength-tunable light source including a semiconductor laser.
Description of the Background Art
In recent times, a wavelength division multiplexing communication system that enables a large capacity transmission with one optical fiber by multiplexing a plurality of signal light beams having different wavelengths has been achieved with significant increase in the demand for communication.
A laser diode (LD) of a single mode (hereinafter referred to as a single-mode LD) in which a high side mode suppression ratio (SMSR) of at least greater than or equal to 30 to 40 dB is obtained is suitable for a light source used in the wavelength division multiplexing communication system. The single-mode LD includes, for example, a distributed feedback laser diode (hereinafter referred to as a DFB-LD) and a distributed Bragg reflector laser diode (hereinafter referred to as a DBR-LD).
To achieve the wavelength division multiplexing communication system, a low-cost wavelength-tunable light source that covers the entire wavelength band is necessary. An LD light source monolithically integrated on the same substrate has been receiving attention for the wavelength-tunable light source.
The wavelength-tunable light source is typically used in combination with an external modulator module that generates a data signal, and furthermore, monolithically integrating an electro-absorption (EA) optical modulator and a Mach-Zehnder (MZ) optical modulator on the same substrate is also researched.
Japanese Patent Application Laid-Open No. 2003-258368, for example, discloses a wavelength-tunable light source having been formed such that an output side of a plurality of DFB-LDs is connected to an input waveguide of a multimode interference optical multiplexing circuit (hereinafter referred to as an MMI), and the light multiplexed by the MMI is amplified by a semiconductor optical amplifier (hereinafter referred to as an SOA) and the amplified light is output from an output waveguide.
Japanese Patent Application Laid-Open No. 2007-65357, for example, discloses a wavelength-tunable light source in which an output side of a plurality of LDs is connected to an N×2-MMI (N is a natural number greater than or equal to three) and two output waveguides of the MMI are connected to two arms of the Mach-Zehnder modulator. According to Japanese Patent Application Laid-Open No. 2007-65357, an optical loss can be reduced compared to the case where an N×1-MMI is used.
Japanese Patent Application Laid-Open No. 2011-44581, for example, discloses a wavelength-tunable light source formed such that an output side of the plurality of DFB-LDs is connected to the MMI, and after a phase is adjusted at each of the two output waveguides of the MMT, the light is output from one output waveguide through the other MMI. According to Japanese Patent Application Laid-Open No. 2011-44581, higher output can be achieved and a signal-to-noise ratio (SN ratio) can be improved.
Japanese Patent Application Laid-Open No. 2003-215369, for example, discloses a configuration of a Mach-Zehnder interferometer including two arm waveguides, the configuration increasing a distance between the arm waveguides so that when only one of the arm waveguides is heated with a heater, the other arm waveguide is prevented from the influence of heat.
Japanese Patent Application Laid-Open No 2005-156855, for example, discloses a configuration of a Mach-Zehnder interferometer including two arm waveguides, the configuration increasing a distance between the arm waveguides so that when only one of the arm waveguides is heated with a heater, the other arm waveguide is prevented from the influence of heat, the configuration having a groove between the arm waveguides.
In a wavelength division multiplexing communication system in which the transmission speed is greater than or equal to 40 Gbps in a main line system, digital coherent communication using an optical phase modulation has been put into practice use in recent times. In the wavelength division multiplexing communication system adopting the digital coherent communication, a narrow-line-width wavelength-tunable light source having a laser oscillation line width of less than or equal to 1 MHz, more desirably less than or equal to 500 kHz, is used as a light source for transmission and reception.
In the wavelength division multiplexing communication adopting the digital coherent communication, separate wavelength-tunable light sources are needed for transmission and reception when the wavelengths of the light signals used for transmission and reception are different from each other, which increases power consumption of the entire transceiver including the wavelength-tunable light sources.
Meanwhile, in the wavelength division multiplexing communication adopting the digital coherent communication, the wavelengths of the light signals used for transmission and reception are often the same, and the narrow-line-width wavelength-tunable light source typically has different light output needed for transmission and reception. In such a case, one wavelength-tunable light source is desirably provided to be used as a light source for transmission and reception in terms of reducing the power consumption and a mounting area of the entire transceiver.
For example, in a case where the wavelength-tunable light source of each of Japanese Patent Application Laid-Open No. 2003-258368, Japanese Patent Application Laid-Open No. 2007-65357, and Japanese Patent Application Laid-Open No. 2011-44581 is used as the light source for transmission and reception, the wavelength-tunable light source of each of Japanese Patent Application Laid-Open No. 2003-258368, Japanese Patent Application Laid-Open No. 2007-65357, and Japanese Patent Application Laid-Open No. 2011-44581 has one output, which needs to be made to two outputs with a polarization maintaining coupler. However, a branch loss occurs in the polarization maintaining coupler in the configuration, and thus a drive current density of the SOA located on the output side of the wavelength-tunable light source needs to be increased to increase the light output from the wavelength-tunable light source in order to compensate for the branch loss. This may lead to the cause of increase in power consumption and a laser oscillation line width. The Mach-Zehnder interferometer of each of Japanese Patent Application Laid-Open No. 2003-215369 and Japanese Patent Application Laid-Open No. 2005-156855 heats only one of the arm waveguides with the heater to change a refractive index of the arm waveguide in order to control a phase, and the Mach-Zehnder interferometer positively adopts the action of heat. Thus, the heat is transmitted to the other arm portion, thereby possibly influencing the refractive index of the waveguide.